The present invention relates to a high-modulus glass composition. More particularly, this invention relates to a glass composition suitable for use as a substrate for information recording media, which is required to have excellent surface smoothness and a high modulus. The present invention further relates to a substrate for information recording media, which comprises the glass composition, and to an information recording medium.
Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium.
However, the media comprising a substrate currently in use are weighed down by themselves and resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity.
For reducing the bending of a substrate medium and diminishing the resonance of the medium being rotated, it is necessary to heighten both the modulus of elasticity (Young""s modulus) of the substrate of the medium and the rigidity thereof which is the value obtained by dividing the modulus of elasticity by the specific gravity.
The aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity of 71 GPa and a rigidity of 26 GPaxc2x7cm3/g. This conventional substrate material, having such properties, hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material, although this goes against the current trend toward thickness reduction in disk substrates for device miniaturization.
In contrast, substrates made of a tempered glass are superior to the aluminum substrate in both modulus of elasticity and specific gravity. For example, a glass substrate obtained by subjecting a commercially available soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPaxc2x7cm3/g.
Also known besides the above one is a glass substrate obtained by tempering commercially available Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPaxc2x7cm3/g, these properties are still insufficient.
A high-rigidity substrate for information recording media which is made of a material other than tempered glasses is on the market. This substrate comprises a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPaxc2x7cm3/g. However, this substrate, after polishing, inevitably has residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, this crystallized-glass substrate has a drawback that it is inferior in surface smoothness to the substrates made of a tempered glass.
Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices and smaller thickness in disk substrates, there is a desire for a glass composition which has further improved properties, i.e., which has a high Young""s modulus and a high rigidity, can be easily tempered, and gives a substrate having high surface smoothness through polishing.
Accordingly, one object of the present invention is to provide a glass composition which has a high modulus of elasticity (Young""s modulus) and a high rigidity (modulus of elasticity/specific gravity) and is capable of being effectively inhibited from bending or vibrating when used as the substrate of an information recording medium.
Another object of the present invention is to provide a substrate for information recording media, which comprises the glass composition.
The present invention has been achieved in view of the above-described problems of prior art techniques and the above-described requirements.
The present invention provides a glass composition comprising the following components in terms of mol %: 40 to 60% SiO2, 8 to 25% Al2O3, 2 to 20% Li2O, 0 to 5% Na2O; provided that the content of R2O (R2O=Li2O+Na2O) is from 2 to 20%, 0 to 10% TiO2, 0 to 10% ZrO2, 5 to 25% MgO, 0 to 25% CaO and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from more than 15 to 40%.
The glass composition preferably comprises the following components in terms of mol %: 40 to 55% SiO2, 10 to less than 20% Al2O3, 2 to 10% Li2O, 5 to 5% Na2O; provided that the content of R2O (R2O=Li2O+Na2O) is from 2 to 10%, 0 to 10% TiO2, 0 to 10% ZrO2, 2 to 25% MgO, 0 to 25% CaO, and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from more than 15 to 30%.
The glass composition preferably has a rigidity as defined by (Young""s modulus)/(specific gravity) of 35 GPaxc2x7cm3/g or higher and a modulus of elasticity as represented by Young""s modulus of 95 GPa or higher.
Furthermore, the glass composition is preferably one which has undergone an ion exchange treatment in at least one molten salt containing ions of potassium, sodium, or both.
The present invention further provides a substrate for information recording media, which comprises the above-described glass composition which has undergone the ion exchange treatment.
This invention still further provides an information recording medium containing the substrate.
The reasons for limitations of the components of the high-rigidity high-modulus glass composition of the present invention are explained below. Hereinafter, unless otherwise indicated, all percents are by mole.
SiO2 is the main component constituting the glass. If the proportion of SiO2 is lower than 40%, the glass has impaired chemical durability. On the other hand, if the proportion thereof exceeds 60%, the desired modulus of elasticity is not obtained. Consequently, the proportion of SiO2 should be from 40 to 60%, and is preferably from 40 to 55%.
Al2O3 is an ingredient which improves the modulus of elasticity and rigidity of the glass and increases the depth of a compression stress layer formed by ion exchange. Al2O3 further serves to improve the water resistance of the glass. If the proportion of Al2O3 is lower than 8%, these effects are insufficient. On the other hand, if the proportion thereof exceeds 25%, the results are an increased viscosity, an increase in liquidus temperature which is severer than the viscosity increase, and impaired meltability. Consequently, the proportion of Al2O3 should be from 8 to 25%, and is preferably from 10 to less than 10%.
Li2O, which is an ingredient to be replaced in ion exchange, serves to improve the modulus of elasticity and rigidity of the glass and to lower the melting temperature of the glass to thereby enhance its meltability. If the proportion of Li2O is less than 2%, the rigidity is insufficient. On the other hand if the proportion thereof exceeds 20%, the substrate has impaired weatherability and impaired acid resistance. Consequently, the proportion of Li2O should be from 2 to 20%, and is preferably 2 to 10%.
Na2O, which is an ingredient to be replaced in ion exchange, serves to lower the melting temperature and the liquidus temperature to thereby enhance meltability. If the proportion of Na2O exceeds 5%, the modulus of elasticity is decreased, and weatherability and acid resistance are impaired. Consequently, the proportion of Na2O should be 5% or less.
If the total amount of Li2O and Na2O (R2O) is less than 2%, ion exchange cannot be conducted, and meltability is insufficient. On the other hand, if the total amount thereof exceeds 20%, the modulus of elasticity and rigidity are decreased. Consequently, the total amount of R2O is preferably from 2 to 20%.
TiO2 is an ingredient which improves the modulus of elasticity, rigidity, and weatherability of the glass. However, if the proportion thereof exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of TiO2 should be 10% or lower.
ZrO2 is an ingredient which improves the modulus of elasticity, rigidity, and weatherability of the glass. However, if the proportion of ZrO2 exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of ZrO2 should be 10% or lower.
MgO is an ingredient which heightens the modulus of elasticity, rigidity, and meltability of the glass. However, if the proportion of MgO is less than 5%, its effect is small. On the other hand, if the proportion thereof exceeds 25%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of MgO is preferably from 5 to 25%.
CaO is an ingredient which heightens the modulus of elasticity, rigidity, and meltability of the glass. However, if the proportion of CaO exceeds 25%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of CaO is preferably 25% or lower.
SrO is an ingredient which heightens the meltability of the glass. However, if the glass contains SrO in a large amount, it disadvantageously has an increased specific gravity. Consequently, the proportion of SrO should be 6% or lower.
If the total amount of MgO, CaO, and SrO (i.e., the amount of RO) is 15% or lower, the glass is insufficient in modulus of elasticity, rigidity, and meltability. If the total amount thereof exceeds 40%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the total amount of RO is preferably from more than 15 to 40%, and more preferably from more than 15 to 30%.
Besides the ingredients described above, other ingredients may be added in a total amount of up to 3% for the purposes of coloring, melt clarification, etc. Examples of such optional ingredients include As2O3, Sb2O3, SO3, SnO2, Fe2O3, CoO, Cl, and F.
This glass composition, which contains Li2O and Na2O, can be easily made to have an increased fracture strength by immersing the composition in at least one molten salt containing ions of potassium, sodium, or both at a temperature not higher than the distortion point of the glass composition to thereby interchange these ions and thus generate a compression stress on the surfaces of the composition.
When this glass composition is used as a substrate for information recording media, this substrate is less apt to bend or suffer resonant vibration because it has a higher modulus of elasticity and a higher rigidity than conventional substrates. Therefore, the recording medium employing this glass composition is especially suitable for use in recording apparatuses of the high rotational speed type.